The present invention relates to the transmission of packet information in the air interface of a packet radio system. The general range of application is any digital cellular system based on TDMA, Time Division Multiple Access.
The majority of current cellular networks provide effective data and speech services based on circuit switched technology. However, the utilization or transmission resources in circuit switching is unoptimal, because the transmission connection is maintained throughout the contact irrespective of the fact whether information is transmitted or not at a given moment. Transmission resources are shared by multiple users, which means that the reservation of circuit switched connection for one subscriber only unnecessarily uses by transmission resources from other subscribers. The burstiness of data services also is a drawback in circuit switched systems. The utilization of the channel can actually be enhanced by applying packet switched information transmission. On the other hand, packet switching should be used only when an actual need arises, because the burstiness of data services is high and this may cause interference in circuit switching.
The future third generation cellular system UMTS (Universal Mobile Telecommunications System) must be able to transmit both circuit switched and packet data transmission, such as ISDN (Integrated Services Digital Network) and ATM (Asynchronous Transfer Mode) transmission. Now the key factor is the air interface, where an advanced multiple access technology is employed; by means of this, the channels supporting different types of services must be effectively multiplexed in the air interface both to and from the radio channel. The conference publication “Mobile and Personal Communications, 13-15 December 1993, Conference Publication No. 387, IEE 1993” includes the article “A Reservation Based Multiple Access Scheme for a Future Universal Mobile Telecommunications System” by J. M. DeVille, which describes the requirements to be set for the air interface of a UMTS system. For example, multiple access must be able to utilize the Inactivity of the Information source by granting a physical channel only when there is activity on the logical channel, and to support different bit rates so that time slots in the frame are allocated to the logical channel according to the needs of the situation.
In order to satisfy these and other requirements, there is suggested the multiple access control method PRMA++ (Packet Reservation Multiple Access), which is part of a design for third generation cellular systems related to the transmission of packetized speech and data. PRMA++ can thus be used as multiple access control bath In packet switched and circuit switched transmission. The PRMA++ method concentrates on using one time slot in the transmission of packet data.
On the radio channel, PRMA++ uses Time Division Multiple Access TDMA. This allows the subscriber to share the transmission resources of the radio channel. The TDMA frame is divided into time slots, where the transmitted burst carries the data as well as signals connected to channel coding, notifications etc. In the uplink direction, which is the direction from the mobile station to the network (base station:), there are two types of time slots: reservation or R-slots, where only channel request bursts are transmitted, and information transmission or I-slots, which are only used for transmitting information bursts. In the channel request burst, the mobile station uses an Air-Interface Channel Identifier containing the network address of the mobile station, which address identifies the logical channel, and where it requests one or more time slots from the frame, according to the needs of the moment. In the downlink direction, i.e. from the network (base station) to the mobile station, there are likewise two types of time slots: I-slots reserved from transmitting information, and acknowledgement or A-slots. When the mobile station requests access to the network, the base station acknowledges the request on A-slots by transmitting the address of the subscriber and the number of the I-slot. From this onward, the said I-slot is reserved for the use of the mobile station.
Let us suppose that the number N of the PRMA++ time slots in one TDMA frame is a system configuration parameter. On the uplink channel, one TDMA frame now contains one R-slot and N-1 numbers of I-slots. All mobile stations start transmission by transmitting a channel request on the R-slot, and if several mobile stations use the same R-slot for transmitting the request, collisions may occur. The downlink TDMA frame includes, in addition to the above mentioned A-slot for acknowledging channel requests transmitted on the R-slot and I-slots, also a fast FP (Fast Paging) slot constituting the fast paging channel, on which the mobile station is notified of incoming data transmission and of information transmission slots.
The mobile station starts transmission by channel request on the uplink channel on an R-slot, which is used for this purpose by all mobile stations of the same cell. The base station acknowledges the received channel request on the acknowledgement burst on the downlink A-slot. If no requests are transmitted on the R-slot, or if on the channel there are collisions, identified by the base station, the base station transmits an idle flag on the acknowledgement burst of the respective A-slot, so that the mobile station understands to repeat the channel request after some time. In case The channel request sent on the R-slot was correctly received, but there are no free time slots for the transmission, the mobile station is notified of this on the next downlink time slot. The mobile station queues for access until a free time slot is found.
The R-slot contains a training sequence, address of the mobile station, number of requested information slots and a circuit switched flag. The flag informs whether the reservation is valid for the duration of the packet or longer. The channel is reserved, until an order for cancelling the reservation arrives. The A-slot acknowledgement burst contains the address of the requesting mobile station as well as the channels that are granted for traffic. The mobile station receives the acknowledgement burst, whereafter it tunes the receiver and transmitter to the allocated channel. Traffic on this channel is started, and it continues as long as there is data or speech to be transmitted. In packet data transmission, the number of bursts—bursts here meaning packets that are transmitted after one channel request—can be constant.
The base station uses the fast paging slot, FP-slot, to notify the mobile station of an incoming packet. The mobile station listens to the FP-channel and decodes all received messages in order to notice its own identifier. The time slot on the fast paging channel contains a list of those I-slots that are allocated for the mobile station. The mobile station acknowledges its own paging by transmitting an acknowledgement in the FP-acknowledgement slot.
According to what was said above, it is characteristic of the suggested UMTS system both in the uplink and in the downlink directions that physical channels are not allocated for connections which are not active at a given moment, and hence they do not reserve capacity in vain. The channels are always reserved by the same protocol, both in the case of circuit switched and packet transmission. The allocation of the channels is not dynamic, wherefore the channels reserved for packet usage cannot easily be altered. Reservation, fast paging and acknowledgement slots are given slots, and the state of the art does not comment on altering these. Moreover, the known method does not pay particular attention to the symmetricity or asymmetricity of packet transmission when creating a transmission channel.